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Britons will have to make a choice about whether their net connection is filtered

Prime Minister David Cameron has announced that UK internet service providers (ISPs) will be putting pornography filters on domestic internet connections.

The speech is the culmination of a long campaign by the government to get ISPs to impose default filters for adult and sensitive subjects. But what will the changes mean in practice?

What does the government want ISPs to do?

The government has been calling on ISPs to filter legal pornography and other adult subjects "by default". This would mean that anyone who wanted to view websites dealing with these sensitive subjects would have to choose to do so.

Before now, ISPs had resisted the call for the filters to be on by default. Their preference had been for an "Active Choice+" system that would make new customers choose what level of filtering they wanted at the account activation stage.

This stance has changed. Instead of letting people choose what filtering they want, the "Yes" option for filtering will be pre-ticked.

The call for filters for legal pornography is distinct from the other steps the government wants ISPs and search engines to take to tackle illegal pornography particularly that involving the sexual abuse of children.

Who will be affected by this?

Initially only new customers of ISPs will be asked to make a choice.

However, the UK's big ISPs have said they will write to all their existing customers asking if they want to turn on internet filters. This is likely to happen in 2014 and, when complete, will reach about 95% of the UK's net using households. Some smaller ISPs have said they will not impose filters.

What filtering system are ISPs using?

It depends because different ISPs use different approaches. Some offer downloads of filtering software that can be installed on every PC or web-browsing gadget in a household. Others prefer a "network level" filter that sees the ISP monitor traffic to block access to sites customers do not want people in their home seeing.

Most ISPs are gradually moving towards the network level filters that do the monitoring and blocking at the router or ISP level. This filters traffic to any device that connects via a home's net connection. The UK's mobile operators have operated network level filtering for many years.

What happens if a customer wants to change what is blocked?

Again, it varies by ISP.

For those that use installed software, the blocking can be turned off or customised on each machine as required.

Most ISPs that operate network level blocking let customers turn it on and off as needed via their account settings page on the web. In some cases customers might occasionally need to call a customer support line to adjust the filter settings.

Some ISPs operate a time-based system that lets parents restrict access to certain sites at certain times. Many people choose to block access to social networks in the early evening when they want their offspring to be getting on with their homework.

How will this change the web browsing experience?

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Net savvy children may be able to get round the filters, warn critics

We do not know for sure. The filters that ISPs are using are supplied by many different companies who have not shared their categorisation systems or lists of which sites fall into which category.

Computer-based filtering systems are notorious for being simultaneously too strict and too lax. Studies of filters on some UK ISPs have shown that well-known porn sites go unblocked while education sites about sexually transmitted diseases or sexual health are inaccessible. The filtering system run by some of the UK's mobile operators are regularly criticised for blocking legitimate sites.

Ironically, some people have reported that the "family friendly" filters now required on public wi-fi hotspots stopped them reading news articles about government plans to impose pornography filters.

Why is the government doing this?

The government says the filters are needed to ensure children are spared the "corroding" influence of pornography. It hopes the filters will stop children inadvertently seeing images or visiting websites their parents do not want them to see.

However, it is not clear what effect it will have on children and young people that go looking for pornography or other sensitive subjects. Studies suggest the filtering systems can be fooled quite easily and present no real obstacle to anyone that can use a search engine.

Does anyone oppose the call for filtering?

Many organisations and people are wary of the policy. ISPs say they should not be seen as moral arbiters and that it is up to parents to police and oversee the web browsing habits of their children. They warn that imposing filters could create "complacency", which might make parents less likely to scrutinise what their offspring do online.

Many people point out that many young people now have their own phones and the material they share is sent from person to person and thus avoids any parental oversight.

In addition, digital rights activists criticise the fact that filtering lists are not exposed to scrutiny. They fear that the lists of "sensitive" subjects will be expanded to gradually stifle dissent. They point to filtering systems in other countries that were started with an expressly moral purpose but were subverted for more political ends.